Fair Value Measurements (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	12 Months Ended
	Mar. 31, 2018	Dec. 31, 1991	Dec. 31, 2017	Apr. 18, 2017	Mar. 31, 2017
Public Preferred Stock [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Public preferred stock par value (in dollar per share)	$ 0.01
Preferred stock dividend rate per annum	12.00%	6.00%
Senior Redeemable Preferred Stock [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Carrying amount of senior redeemable preferred stock					$ 2.1
Redemption amount of senior redeemable preferred stock				$ 2.1
Carrying (Reported) Amount, Fair Value Disclosure [Member] | Public Preferred Stock [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Public preferred stock	$ 132.5		$ 131.6
Estimate of Fair Value, Fair Value Disclosure [Member] | Public Preferred Stock [Member]
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Public preferred stock	$ 40.2		$ 42.2